1. Field of the Invention
The invention generally relates to a circuit substrate, and more particularly, to a circuit substrate having a three-dimensional (3-D) grounding circuit loop.
2. Description of Related Art
In order to maintain signal integrity and power integrity, a conventional circuit substrate for transmission of high speed signals typically adopts a full reference plane. This is done so that when an electronic signal is transmitted in a signal line, a characteristic impedance of the signal line is kept constant. A good impedance matching design between a control terminal and a device terminal is crucial to reduce the insertion loss and the return loss due to the impedance mismatch, especially when the signal is transmitted in a high speed and a high frequency environment. Accordingly, the quality of the signal transmission will not be affected.
Generally speaking, the reference plane for a high speed signal may be a power plane or a ground plane. A plurality of signal lines is disposed on a side of the reference plane, and a line width and a thickness thereof are kept constant, so that the characteristic impedance of the signal line can be kept constant. However, the size of an area occupied by the reference plane affects the wiring space available for the signal lines, especially for a double-layered circuit substrate. For this kind of circuit substrate, in order to maintain a full ground plane, signal lines can only be disposed on a single surface. Therefore, the conventional doubled-layered circuit substrate design cannot satisfy needs for high density and high speed wiring. Moreover, when an operating frequency increases, typically a terminator is employed to reduce the reflection generated in a high speed signal transmission due to impedance mismatch, so that a normal signal can be accurately and completely transmitted from one terminal of the signal line to another.